1. Field of the Invention
The present invention relates to microelectromechanical systems (MEMS) devices and methods of fabricating such devices.
2. Description of the Related Art
MEMS comprise a class of very small electromechanical devices that combine many of the most desirable aspects of conventional mechanical and solid-state devices. Unlike conventional electromechanical devices, MEMS can be monolithically integrated with integrated circuitry while providing both low insertion losses and high electrical isolation. Two main categories of MEMS are actuators and sensors. MEMS actuators can be very precise because they perform only a small amount of work on their environment. MEMS sensors are virtually non-invasive because of their small physical size.
MEMS devices typically comprise a fixed element attached to an electrically insulating substrate and a suspended element with a substantial portion free from mechanical attachment to the substrate so as to be movable with respect to the fixed element.
High sensitivity MEMS devices require high aspect-ratio structural, elements. Deep etched structures of about 2 micrometers (μm) wide and from about 20 μm to about 100 μm deep with a narrow spacing, for example, about 2 μm, between adjacent features are often needed to achieve the desired sensitivity. In addition, if, for example, these devices are also required to provide high electrical voltage isolation, an insulating material must be used to electrically isolate and mechanical join various conductive members within the device. Lithography and patterning over the aforementioned high aspect-ratio MEMS devices is thus required, and presents a major challenge in the fabrication of such devices.
For example, U.S. Pat. No. 6,159,385, issued to the assignee of the present invention and incorporated herein by reference for its teachings of a low temperature MEMS fabrication process and various MEMS devices, discloses a high aspect-ratio MEMS device having a movable element such as a cantilevered beam suspended over a first substrate, preferably of glass. A second substrate is fabricated by growing a device layer of doped silicon on a silicon sacrificial or handle layer and depositing a layer of insulating material such as silicon dioxide on the device layer. With the insulating layer in confronting relationship with the glass substrate, the two substrates are joined using an adhesive bonding agent to create a composite structure. The handle layer is then removed exposing the top side of the device layer which is patterned and etched using anisotropic plasma dry etching techniques to define the MEMS elements. This etch stops upon reaching the insulative silicon dioxide layer. A second mask is then applied to pattern the silicon dioxide layer from the top to define an insulating bridge. Finally, an oxygen plasma etch is performed to undercut the adhesive bonding agent and thereby release the movable MEMS element.
The two-mask process described above provides the desired insulating joinder structure or bridge for mechanically coupling and electrically isolating the MEMS elements. It will be noted, however, that, as stated earlier, patterning and etching of the silicon dioxide layer over the deeply etched MEMS structure is required, a process that is difficult to perform with consistent accuracy, resulting in low yields.